Signaling system



April 25, 1933. J. w HORTON 1,905,714

SIGNALING SYSTEM Filed April 10, 1931 2 Sheets-Sheet 1 FIG.

//v VEN TOR J W. HOR TON A TTO/PNEV 2 Sheets-Sheet 2 Filed April 10 1931 N mm R m0 VH w ATTORNEY PatentedApr. 25, 1933 crosnrn w. HORTON, or cA BnInsn, missaonusnrrs, :ASSIGNOR ro BE rnnnrnonia LABORATORIES, rnconronnrnn, or new YORK, 1v. n oonronnr on or new our: 7

SIGNALING sYsrm/r Application filed Aprillt), 1931. Serial 1o.:529,o4s.

This invention relates to signaling systems 7 and more particularly to means whereby a signal band, extending from zero frequency to a frequency of considerable value, maybe effectively transmitted to and faithfully reproduced at a receivingstation.

Systems of this general type have previously been disclosed. In one such system 1nvolving electro-opticaloperation, an image current band is separated into two parts by filters. One part, including the higher frequency current components, is amplified and supplied to the transmission circuit. The second part, comprising the low frequency current componenta-is used to modulate a carrier wave, and themodulated wave is supplied to the transmission circuit. At the receiving station, the modulated wave'is detected to yield the low frequency current components, which are passed through attenuat 'ing and phase correcting networks to compensate the distortion introduced by thefilters at the transmittingstation and thecompensated components are combined with the directly transmitted part to reproduce the signal band originated at the transmitting station. 7 r p In another prior system the image current band is usedto modulate a carrier wave, a filter selects one of the resulting sidebands andthis sideband is transmitted; At thereceiving station, the incoming-sideband is combined'with oscillations to produce the image current wave mitting station. Both of the above. described systems were devised for the purpose of producing a wave extending over a frequency band which lies within the transmission range of the transmitting circuit and which permits the image current band to be faithfully reproduced at the receiving station. 7

In certain signaling systems. the signal current produced comprises a band which extends continuously from zero to an upper limiting frequency, the various components having amplitudes which are insufiicient for transmission to a distant station or for effect- 'lica of thesignalbandoriginated at the trans combined with local oscillationsto'step it terns requirethe use of a direct currentamplifier. Moreover, in order that the signal currentmay be raised to-the energylevel required? to effect modulation, it has been found that multi-stage amplifiers must be used...

Although direct current amplifiers can be successfully operated, their use necessitates very special care' in the design{ of the/circuit and they-must be constantlysupervised in ordcrto ensure stable operation. 'When a direct current amplifier including a number of cascade connected stages is used, the design and operatingrequirements are especial ly burdensome. Forv these reasons, theu se of directcurrent amplifiers is undesirable.

An object of the present invention isto enable. signal systems of the type described above to be successfully operatedwithout requiring the use ,of :direct current amplifiers. Anotherobjectjis to permit-an alternating currentadevicetobe used to repeat signal current. bands, having direct current components, and to reproduce anundistorted rep- Onetypical embodiment of the presentinvention may comprise an electro-opticaltransinitting terminal apparatus including an-interrupter ,for chopping a light beamf supplied to a photo-electric cell, which is thereby activated to produce a-composite current consisting of two sections, a transformer coupled device for amplifying the composite current. produced 7 by the photo-electric cell, filters for selectively, separating thetwo sections of the composite photo-electric current, one of whichis amplified and supplied to the transmission circuit, while the other is down to a frequencysuch that its mid-frequency is slightly abovefthe upper limiting frequency of'the firstsection, and afilter for selecting a mid-portion of the second section and having a -.cut.-ofi characteristic adapted to distort the 'componentsincluded in the'selected portion. p l

In the operation of the transmitting apparatus, the alternating current amplifier will repeat without. distortion the higher frequency components of the first section, which is identical with the image current, but will distort certain of its components in the neighborhood of zero frequency. The amplifier will also repeat the second section which comprises two sidebands I extending continuously over a frequency rangewhich corresponds to twice the frequency of the first section and includes the rate at which the light beam is chopped,ras its mid-frequency. The locally supplied oscillations combine with the second section to step it down to a point in the frequency spectrum such that its mid-frequency'lies slightly above the upper limiting frequency of the first section, the co-operating filter-selects the midportion of the second section, which includes frequency components corresponding to those of the first section distorted by the amplifier. The cut-off characteristic of this filter is such, that :it distorts the several current components of the selected portion in the same manner as the alternating current amplifier distorts the original current components in the neighborhood of zero frequency. Since the selected portion extends over a very'limited range which lies closely adjacent the upper limiting frequency of the first section, the total frequency range occupied by the currents supplied to the transmission circuit is only slightly greater than that of the image cur rent band. i

At the receiving station, thetwo portions of the incoming energy are separated by filters. The mid-portion is combined with local oscillations to step it down in frequency, so that its mid-frequency component is zero.

After selection by afilter, the components derived from .the'sidebandsand the distorted approximately zero frequency components of the first section are combined in such manner that the output current of the amplifier is a faithful reproduction of the photo-elec-, tric current producedat the transmitting station.

A more detailed description of the invention follows and is illustrated inthe'attached drawings.

Fig. 1 diagrammaticallyillustrates a system embodying this invention, and

Figs. 2 to 8 illustrate curves which will be used toexplain'the operation of various units of Fig. l. j

Transmitting apparatus T includes a light source 1 of constant intensity, a suitable lens 2 for focusing light rays from the source 1 upon an apertured or slotted section of 'thedisc 3, adj acent its periphery. This disc is rotated to cause the light beam to pulsate or be chopped at a carrier frequency rate. In other words, source 1, lens 2 and disc 3 cooperate to produce a carrier lightbeam.

Lenses 4 and 5 andapertu'red members 6 and 7 cause the carrier light beam to be focused on a small elemental area of the picture 8, which is shown as a film of varying transparency mounted on a transparent drum 9. In accordance with standard practice in picture transmission systems, the drum is given a motion of rotation and of translation to cause the light beam to successively scan the elemental areas of the entire picture. Preferably the picture is scanned as disclosed in Patent 1,706,032, 19, 1929, of ll/LB.

Long. 7

The light beam passes through the picture to a light sensitive cell 10 which is thereby activated to cause varying electric currents, i. e., a signal or picture current band, to flow through the primary winding of transformer 11. The current flowing through the primary of transformer 11 maybe mathematically represented a Fourier series which includes a term corresponding to zero frequency, aterm of signal frequency, in addition to terms corresponding to signal sidebands 7 about the chopping frequency and possibly higher order sidebands about harmonics of the choppin frequency. Except for-the harmonics, whic are of negligible amplitude, these currents consist of two major'groups of components,

one group extending continuously from zero frequency to an upper limltlng frequency r representing the picture currents, and the other, comprising aband having the frequency of the carrier beam as its mid-'fre quency and extending continuously in the frequency spectrum on both sides thereof to a in At any given instant the envelope of the cur- 1 rent in this second group is' proportional to the light transmitted by the picture. 7

In Fig. 2' the ordinates of the dotted curve represent attenuation, line curves represent current while the abscissae represent frequency. Curve a represents the attenuation-frequency characteris- .tic of the transformer coupled amplifier 1112. Curve?) represents thecurrents of the first group or picture currents, and curve 0 represents currents of the second group or sidebands, both of which flow through the primary of the transformer 11. As shown by curve a, the transformer 11 presents substantially infinite attenuation to current components of zero frequency, a gradually decreasing attenuation for current components of increasing frequencies extending from adjacent zero frequency to an upper limiting value, and substantially Zero attenuation for current components of the higher frequencies. Because of theattenuation of the transformer, current componentsof zero frequency will be entirely suppressed, those of a conand those of the full 7 tinuous narrowsection in the vicinity of zero frequency will be distorted more or less'as.

their frequency-departs from zero, and the remainder of the current components will be induced without distortion in the winding of the transformer 11. The picture current, withsome components secondary distorted, and the sidebands," asthey are induced in the secondary winding of transforr n er 11,"are indicated by full line curves b and 0 respectively, of Fig. 3. a The induced picture current and sidebands are amplified by the device 12, the output circuit of whiclii'n- 'cludes a filter 13 havinga frequency-attenuation characteristic represented by the dotted curve e and a filter 17 having a frequencyattenuation characteristic represented by the dotted curve a 7 Amplified picture currents, represented by full line curve 6 are selected by the filter "13, amplified by the amplifier 14;,again selected by the filter 15 and supplied to the transmission circuit '16. Filter 15 has a frequency-attenuation characteristic represented by dotted curve a, in Fig. 4: and hence 'it" operates to supply picture currents, inditransmission line 16. i 1 The sidebands are selected by the filter 17 and impressed upon the modulator 18, in

cated by full line curve 6 of'F-ig. 4, to the which they are combined with oscillations supplied by the oscillator 19. The frequency of these oscillations is such that the sidebands are stepped down to a position in the frequency spectrum indicated by the full'line curve 0 of Fig. 5. The current band represented'by the curvec is supplied to the filter 20, which is designed to select the current components thereof on bothsides of its midfrequency, i.-e., those which correspond to.

the components attenuated by the transformer 11', and to have a cut-oil characteristic 0" which attenuates the respective components on opposite sides-ofthe mid-frequency to the-same extent as the transformer 11; attenuated similar components of the picture band. i l i From Fig. 6 it will be seen that the 'lowest frequency component of the selected portion v :0 of the stepped-down sidebands, occupies a position in the frequency spectrum immegether to constitute the input circuit of an 7 in Fig.2.

currentband to the "light valve '27 g V Picture currents, represented by the curve 6 are selectedby' the" filter 21, amplified by the device 22 and supplied to the amplifier 26.

amplifier 26 ."adaptedto supply the picture The. sideband portion representedby the curve cg is selected by the filter 23 and supplied to the modulator 24 in whichit is combined with oscillations supplied by'the' oscillator 28. The frequency of these oscillations is such'that, when theyare combined" with the sideband portion, the latter is stepped down in frequency (to a position in the frequency spectrum such thatthe mid-frequency component is at zero frequency as indicated by curve 0 The stepped-down portion is selected by the filter 25 and is supplied to the amplifier 26. The current components of the stepped-down portion, represented by the curve 0 in Fig. 7, "are combined in the am plifier26 with the picturecurrent b supplied by the amplifier22-to produce-a picture current band'represented by .thefullline curve 7 6;, in Fig. 8, whic l1 isidentical with curveb The combined amplified' picture currents supplied by the amplifier '26 are impressed upon the light valve=27;

. The light valve herein-shown is designed and operated in a manner disclosed in Patent 1,667,805, May 1,- 1928, of H. E. Ives. The amplified picture currents supplied, by. the amplifier'26 are applied to therribbon of the lightvalve 27 to-control the amount of light transmitted from a steady light source 33 through the optical system represented by the lens 29, aperturedmember 30 and 31, and lens 32, which latters'erves to "focus the light r in the form of a' beam on the unit areas-of a photosensitive "filin- 34'- inounted on driun 35. In accordance withstandardpracticethe drum 35 is-moved in 'synchronismandfinphase with drum 9 at the transmitting-station..-

5 =1 Any well known typeof distortionless amplifier and-any suitable design of'modulator may be used, butthosejincluding space dis charge devices are preferredJThe filters may be of the type disclosedin Patent 1,227 ,114, May 22,1917, to Campbell. 4

The present invention therefore provides a system in which alternating current amplifiers of'standard design are used to amplify currents corresponding to elemental areas 7 of a picture, without requiring the ,trans-.

mission of currents which occupy a materialband per se, andin which an undistorted 'ly greater frequency. range than the picture picture current band is produced at the receiving station.

If desired,'the modulated wave section may be selected, its components distorted in" ponentsJmay be combined with the correspondingdistorted components of the" signal current band at the V-transmitting station,

the manner described above and these Icom- V fun whereby anf amplified undistorted, signal band is supplied to the transmission circuit. Inthis case, the transmission circuit 16 will be directly connected to the light valve 27 at the receiving station. c

While the invention is disclosed as applied to a picture transmission system, it is not limited tosuch use, since its principles may be utilized wherever desired-to eft'ectthe result ohtainahlebv the means or in accordance with the method defined in certain of the appended claims. 5

What is claimed is: y c p a 1; The methodwhich comprises producing two currents having the same signal modulations but occupying different. frequency ranges, selecting a portion of one of said currents and stepping the selected portion into the frequency range of the other of saidcurrents, and combining the selected portion with-the said other. current to selectively modifythe amplitude of certain freq-uencyicompon'entsof said other current.

2. Signaling apparatus comprising means for producing a current modulated in accordance with a signal, means .for'producing a second current modulatedwiththe same signal but having frequency components outside the range of the components ofsaid first signal current, means for selecting components of one of said'currents and stepping the related components intoa desired portion of the frequency range of the-other of said currents. means for combining said selected portion with said other current to selectively change the amplitudes of certain frequency components of said other current;

3. Signaling apparatus comprising means for producing a direct current 'modulated in accordance with a signal, means for producing 'an' alternating current modulated with the same signal but comprising a frequency component outside the range of the components of the modulated'direct current,

. means for selecting components of the modulated alternating current and stepping them into a desired portion of the frequency range of the modulated direct current, means for combining said selected components with certain components of the modulated direct current, whereby the amplitude of said certain components are selectively changed in value.

4. An electro-optical transmitter comprising means 'for producing a composite wave including image currents varying in accordance with the tone values of elemental areas of a field of view and extending over a frequency band from zero to an upper limiting value and a carrier wave modulated. by said image current band. a transformer coupled device for amplifying said band and modulated wave, and means for supplying said band and the mid-portion of said modulated wave to a transmlsslon circuit. 7

5. An electro-optical transmitter as dethe last mentioned means includes a filter.

having a cut-off characteristic such that it distorts the current components of the selected portion of the modulated wave. v '7. An electro-optical transmitter as described in claim l characterized by this that the last therein mentioned means includes a filter which selects the current components of the modulated wave and distorts said com ponents to the same extent as the corresponding components of the image current band are distorted by the transformer coupled amplifier. V 8. Anelectro-optical transmittercomprising means for producing two currents having the same signal modulations but respectively occupying difierent frequency ranges, means for'selecting the mid-portion of one of said currents, means for translating said mid-portion to a position in the frequency spectrum having its lowest frequency component slightly spaced from the upper limiting frequency of the other current, andmeans for supplying said other current and said mid portion to a transmissio'ncircuit. I a

9. An electro-optical system comprising the transmitter defined in the preceding claim in combination with a receiver including means for translating the incoming mid-portion of the modulated wave to a point in the frequency spectrum such that one of its current components is at zero frequency. 10. An electro-optical system comprising the transmitter defined in claim 8 in combination with a receiver including means for translating the'current components ofthe mid-portion of the modulated wave to. a point in the frequency spectrum such that one of its current components is at zero frequency, and means for combining the translated current components with the incoming other current. t v

11. An electro-optical system comprising means for producing a composite wave including an image current varying in amplitude in accordancewith the tone values of elemental areas of a field of view and extending over a frequency band for zero to an upper limiting frequency and a carrier current modulated by said image current band, a transformer coupled device for amplifying said band and said modulated wave, a filter for selecting the'mid-portion of said modulated current, means for transmitting said band and said mid-portion of the modulated current, and means for receiving the transmitted currentsincluding means for, translating the mid-portion of the modulated current to. a

. ducing a composite Wave including an image 7 current band extending from zero to an upper position in the frequency spectrum such that one of its components is at zero frequency.

12. An electro-optica-l system as described in the preceding claim in which the receiver includes means for combining the frequency portion with the image cur translated Wave rent band.

13. An electro-optica-l transmitter, a light sensitive device, meansfor applying to said light sensitive device a light beam chopped at a definite frequency and controlledby elementalareas of the scanned field for pro limiting frequency and a carrier Wave modulated by said image current band, an alternating current amplifier for amplifying said band and said modulated Wave, means for selecting and supplying the amplified band to a transmission circuit, means for selecting said modulated Wave,

portion of said frequency translated: Wave and supplying said selected portion to the transmission circuit.

In Witness whereof, I hereunto subscribe my name this 6th day of April, 1931. JOSEPHW. HORTON.

translating the fre-; quencythereof so that'its carrier component 7 is slightly above the upper limiting frequency of said band, and means for selecting the mid- 

